The present invention relates to semiconductor devices and, more particularly, to a power MOS device having increased channel width and to a process for forming same.
The recent proliferation of portable, battery-powered electronic communication devices has increased the need for low voltage, low on-resistance power MOSFETs for efficient power management. For low voltage MOSFETs, the channel resistance is a large component of the overall on-resistance. Therefore lowering the channel resistance results in a corresponding reduction in on-resistance.
FIG. 1 schematically depicts a prior art device 100 having a planar DMOS stripe configuration on a substrate 101 having a doped upper layer 102. Upper layer 102 includes doped P-well regions 103 and heavily doped N+ source regions 104. On an upper surface 105 of upper layer 102 is a gate region 106 that includes an insulating layer 107 and a conductive layer 108.
One means for reducing channel resistance in a prior art device such as 100 is to increase its channel density in the region 109 of layer 102 underlying gate region 106. Increasing channel density, however, would require a reduction in device geometry and/or a process modification that may be subject to equipment and technique limitations. The present invention offers a desirable alternative to the increased channel density approach for achieving reduced on-resistance in a power device.
The present invention is directed to a power MOS device that has increased channel width and comprises a semiconductor substrate and a doped upper layer of a first conduction type disposed on the substrate. The upper layer comprises a plurality of doped well regions of a second, opposite conduction type and a plurality of heavily doped source regions of the first conduction type at an etched upper surface of the upper layer that comprises parallel corrugations disposed transversely to the source regions. A gate that separates one source region from another comprises an insulating layer and a conductive material. The corrugations provide an increase in width of a channel underlying the gate.
Further in accordance with the present invention is a process for forming a power MOS device having increased channel width on a semiconductor substrate having a doped upper layer of a first conduction type. A stripe mask is formed on an upper surface of the upper layer, and the upper surface is selectively etched to form a corrugated surface comprising a plurality of parallel corrugations. Following removal of the stripe mask, an insulating layer is formed on the corrugated surface, and an overlying conductive layer is formed on the insulating layer, the insulating and conductive layers comprising a corrugated gate region disposed transversely to the parallel corrugations of the upper surface. A dopant of a second, opposite conduction type is implanted to form a doped well region in the upper layer, and a dopant of the first conduction type is implanted into a portion of the corrugated surface adjacent to the gate, thereby forming a heavily doped source region in the upper layer.
In an alternative procedure for forming a gate, a gate trench having a floor comprising parallel corrugations that substantially correspond to the corrugations in the upper surface is etched into the upper layer. Following lining of the trench floor and sidewalls with an insulating layer, the trench is substantially filled with a conductive material to form a gate trench. A dopant of the first conduction type is implanted into a portion of the corrugated surface adjacent to the gate region, thereby forming a heavily doped source region in the upper layer.
The present invention provides a means of increasing channel width in a power MOS device without requiring reduced device geometry. The increased width of the channel is a consequence of the corrugated surface of its overlying gate region.